1. Field of the Invention
The invention relates to a valve arrangement for intermittent application of a liquid adhesive, in particular a high-polymer thermoplastic material, to a substrate.
2. Description of the Prior Art
Valve arrangements generally have a valve body comprising a bore therein terminating in an outlet nozzle. High-polymer materials are supplied via a conduit to the bore. For opening and closing the outlet nozzle a valve needle reciprocates in the bore and must be actuated very exactly, i.e. at exactly defined instants, to ensure the accuracy of the material application necessary when operating at high operating speeds.
For displacing the valve needles, various drive means have been proposed; thus, it is known from German Auslegeschrift No. 1,652,298 to attract the valve needle by means of an electromagnet and thereby to open the outlet nozzle, while the movement of the valve needle into the closed position is effected under the action of a spring. However, the desired exact opening and closing times cannot be implemented by these means.
A variant of this basic principle is apparent from U.S. Pat. No. 4,320,858. In this case the opening of the outlet nozzle is effected with pressurized air and the closure takes place under the action of a spring. However, due to its mass inertia a spring does not respond rapidly enough. Moreover, in the course of time its spring characteristics change due to "fatigue phenomena" and consequently unacceptable fluctuations in the operating rhythm occur.
A similar embodiment for a double valve is disclosed in published European patent application No. 0,111,850.
German patent specification No. 3,200,470 discloses a valve needle comprising a step or differential piston. The valve needle is brought into the opened position by application of compressed air and brought into the closed position by a spring. In this case as well the desired, exactly defined timing of the operating rhythm cannot be achieved.
Finally, a valve arrangement of this type is disclosed in U.S. Pat. No. 3,420,208. In this case the valve needle is provided with a disk-shaped slider piston which is subjected alternately on either side to compressed air. The valve needle is being brought thereby into the opened or closed position. Due to the alternating subjection of the two sides of the piston of the valve needle to compressed air at substantially identical pressures the desired, extremely short valve clearance, which in the extreme case lasts only up to 10 ms, cannot be achieved in this case ether because the movement of the valve needle in one direction does not start until the corresponding pressure on said side of the slider piston has been substantially removed. This results, however, in pronounced fluctuations in the operating rhythm which manifest themselves especially in a particularly unfavourable manner when several such valve arrangements are arranged juxtaposed to each other and must open and close synchronously by joint actuation as is, for example, necessary for the synchronous application of a plurality of punctiform coatings onto a substrate. In these cases fluctuations arise in the operating rhythms of the individual valve arrangements which lead to displacements in the application instant and thus to coating errors with the high transport speeds of the substrate usual today.